Morphogen

A morphogen is a signaling molecule that governs tissue patterning by forming a concentration gradient across a developing tissue and eliciting different cellular responses at different concentrations. The resulting positional information enables cells to adopt distinct fates according to thresholds encoded in gene regulatory networks.

The concept was introduced by Lewis Wolpert in 1969 and popularized by the French flag model, which imagines a gradient of a morphogen that segments a tissue into discrete domains with distinct fates.

Morphogen gradients arise when a source secretes the molecule, which diffuses and is degraded or internalized, creating a smooth concentration profile. Cells sense the local concentration via receptors and downstream signaling pathways, translating it into gene expression changes. The tissue pattern is shaped by thresholds and by cells' developmental competence.

Prominent examples include Bicoid in the early Drosophila embryo setting anterior-posterior identity, Sonic hedgehog (SHH) in vertebrate limb buds and neural tube patterning, BMP and Wnt signals contributing to dorsal-ventral and axial patterns, and Dpp in flies. These morphogens often act through conserved signaling cascades to regulate transcription factors.

Interpretation of gradients depends on receptor expression, feedback, and cross-talk with other signals; gradients can be modulated by transport mechanisms beyond simple diffusion, such as cytonemes, endocytosis, or extracellular matrix interactions. The idea remains central, but real embryos show robustness and complexity.

Morphogen research informs developmental biology, congenital defect studies, and tissue engineering, where establishing controlled positional information is a key goal.